U.S. patent application number 10/658606 was filed with the patent office on 2004-04-15 for bragg grating optical fiber.
Invention is credited to Bagnasco, Mara, Gusmeroli, Valeria.
Application Number | 20040071432 10/658606 |
Document ID | / |
Family ID | 32011024 |
Filed Date | 2004-04-15 |
United States Patent
Application |
20040071432 |
Kind Code |
A1 |
Bagnasco, Mara ; et
al. |
April 15, 2004 |
Bragg grating optical fiber
Abstract
The present invention provides an optical fiber providing high
photosensitivity in the absence of hydrogen loading as well as a
low numerical aperture. One aspect of the present invention relates
to an optical fiber including a core, the core comprising silica
doped with at least about 6 mol % germania and at least about 0.9
wt % fluorine; and a cladding surrounding the core. The optical
fiber of the present invention is suitable for the production of
fiber Bragg gratings.
Inventors: |
Bagnasco, Mara; (Savona,
IT) ; Gusmeroli, Valeria; (Milano, IT) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
|
Family ID: |
32011024 |
Appl. No.: |
10/658606 |
Filed: |
September 8, 2003 |
Current U.S.
Class: |
385/142 ;
385/37 |
Current CPC
Class: |
C03C 25/6226 20130101;
G02B 6/443 20130101; C03C 2201/06 20130101; C03C 13/046 20130101;
C03C 2201/12 20130101; G02B 6/02114 20130101; G02B 6/02123
20130101; C03C 2201/31 20130101 |
Class at
Publication: |
385/142 ;
385/037 |
International
Class: |
G02B 006/16; G02B
006/34 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 10, 2002 |
EP |
020792354 |
Claims
1. An optical fiber comprising a core comprising silica and a
cladding surrounding the core characterized in that the core is
doped with at least about 6 mol % germania and at least about 0.9
wt % fluorine.
2. The optical fiber of claim 1, wherein the core is doped with at
least about 7 mol % germania.
3. The optical fiber of claim 1 or claim 2, wherein the core is
doped with at least about 1.2 wt % fluorine.
4. The optical fiber of any one of the preceding claims, wherein
the core is substantially devoid of boron.
5. The optical fiber of any one of the preceding claims, wherein
the core includes no other dopants in substantial amounts.
6. The optical fiber of any one of the preceding claims, wherein
the optical fiber has a numerical aperture of less than about 0.22
at 1550 nm.
7. The optical fiber of any one of the preceding claims, wherein
the core exhibits an index change of at least about
5.5.times.10.sup.-4 at a wavelength of 1550 nm when exposed to a
dose of radiation having a wavelength of 244 nm and an energy of
428 J through a phase mask yielding an interference pattern with a
visibility of about 80%, said exposure being performed without
hydrogen loading of the optical fiber.
8. The optical fiber of any one of the preceding claims wherein the
core exhibits a ratio of index change at 1550 nm to numerical
aperture of at least about 3.0.times.10.sup.-3, the index change
being caused by an exposure in the absence of hydrogen loading to a
dose of radiation having a wavelength of 244 nm and an energy of
428 J through a phase mask yielding an interference pattern with a
visibility of about 80%.
9. The optical fiber of any one of claims 1-6, wherein a Bragg
grating is present in the core of the optical fiber.
10. The use of the optical fiber claimed in any one of claims 1-5
in a method of fabricating a fiber Bragg grating comprising
exposing a section of the optical fiber to patterned UV radiation,
thereby writing the grating in the core of the fiber.
11. The use claimed in claim 10 in which the said section is so
exposed without hydrogen-loading of the fiber.
12. A method of fabricating a fiber Bragg grating, the method
comprising the steps of providing an optical fiber comprising a
core, the core comprising silica doped with at least about 6 mol %
germanium and at least about 0.9 wt % fluorine, and a cladding
surrounding the core; and exposing a section of the optical fiber
to patterned UV radiation, thereby writing the grating in the core
of the fiber.
13. The method of claim 12, wherein the exposure is performed
without hydrogen loading of the fiber.
14. The method of claim 12 or claim 13, wherein the core of the
optical fiber is doped with at least about 7 mol % germania.
15. The method of an one of claims 12-14, wherein the core of the
optical fiber is doped with at least about 1.2 wt % fluorine.
16. The method of any one of claims 12-15, wherein the core of the
optical fiber is substantially devoid of boron.
17. The method of any one of claims 12-16, wherein the core of the
optical fiber includes no other dopants in substantial amounts.
18. The method of any one of claims 12-17 wherein the optical fiber
has a numerical aperture of less than about 0.22 at 1550 nm.
19. The optical fiber of claim 1, wherein the cladding comprises a
material selected from the group consisting of substantially
undoped silica, germania-fluorine co-doped silica, and
phosphorus-fluorine co-doped silica.
20. The optical fiber of claim 1, wherein the optical fiber has a
numerical aperture of less than about 0.16 at 1550 nm.
21. The optical fiber of claim 1, wherein the core exhibits a ratio
of saturated index change at 1550 nm in the absence of hydrogen
loading to numerical aperture is at least about
9.0.times.10.sup.-2
22. The method of claim 12, wherein the cladding of the optical
fiber comprises a material selected from the group consisting of
substantially undoped silica, germania-fluorine co-doped silica,
and phosphorus-fluorine co-doped silica.
23. The method of claim 12, wherein the optical fiber has a
numerical aperture of less than about 0.16 at 1550 nm.
24. An optical fiber comprising A core, the core comprising silica
doped with at least about 6 mol % germania and with fluorine; and a
cladding surrounding the core, wherein the optical fiber has a
numerical aperture of less than about 0.22 at 1550 nm.
25. The optical fiber of claim 24 wherein the optical fiber has a
numerical aperture of less than about 0.16 at 1550 nm.
26. The optical fiber of claim 24 wherein the core is doped with at
least about 0.9 wt % fluorine.
27. The optical fiber of claim 24 wherein the core is substantially
devoid of boron.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to optical
communications, and more specifically to optical fibers suitable
for the fabrication of fiber Bragg gratings.
[0003] 2. Technical Background
[0004] A high performance optical telecommunication system carries
high data rates over long distances with no electronic
regeneration. For example, rates of 10 Gb/s or more over
unregenerated distances of three to five hundred kilometers have
been achieved. A high performance system may employ high power
signal lasers, optical amplifiers, dispersion compensation devices,
optical switching devices, and may use wavelength division
multiplexing. Optical telecommunications systems are progressing
toward higher speeds and longer span lengths, making the
requirements for system components more and more arduous.
[0005] One such system component is a fiber Bragg grating. A fiber
Bragg grating is formed from a periodic modulation of the
refractive index of the core of an optical fiber. Fiber Bragg
gratings act to selectively retroreflect a single wavelength from a
band of wavelengths propagating in an optical fiber. Fiber Bragg
gratings have found utility in diverse applications such as laser
stabilization, wavelength division multiplexing, gain flattening of
amplifiers, and dispersion compensation.
[0006] Fiber Bragg gratings are conventionally fabricated by
exposing an optical fiber with a photosensitive core to a pattern
of UV radiation having a desired intensity modulation. The pattern
of the UV radiation is generally formed using interferometric
techniques, such as by passing the radiation through a phase mask.
In order to fabricate an effective grating, it is desirable to have
an optical fiber having a high photosensitivity. Conventional
photosensitive optical fibers have a relatively high concentration
of germania in their cores. While increasing the germania content
acts to increase the photosensitivity of the core, it also acts to
increase the refractive index of the core, and therefore the
.DELTA. and the numerical aperture of the optical fiber. Optical
fibers having high .DELTA. and numerical aperture tend not to
couple well to standard single mode optical fibers. As such, fiber
Bragg gratings written in conventional optical fibers having a high
germania content in the core may have high coupling losses to other
optical fibers, and therefore be disadvantaged for use in optical
telecommunications systems.
[0007] Another method of increasing the photosensitivity of a
germania-containing optical fiber is to load the optical fiber with
molecular hydrogen under conditions of high pressure. While
hydrogen loading is a useful method in the fabrication of fiber
Bragg gratings, it adds the extra process steps of hydrogen loading
and post-exposure annealing and requires the use of high pressures
of a highly flammable gas.
[0008] Conventional photosensitive optical fibers do not provide
for the manufacture of optical fiber Bragg gratings with the
desired performance and simplicity of manufacture. There remains a
need for an optical fiber that exhibits high photosensitivity and
desirably low numerical aperture. From the cost and process point
of view, it is further to have a photosensitive optical fiber that
may be used in the manufacture of fiber Bragg gratings without
hydrogen loading.
[0009] Definitions
[0010] The following definitions are in accord with common usage in
the art.
[0011] The refractive index profile is the relationship between
refractive index and optical fiber radius.
[0012] Delta, .DELTA., is the relative refractive index percent,
.DELTA.=(n.sub.i.sup.2-n.sub.c.sup.2)/2n.sub.c.sup.2, where n.sub.i
is the specified refractive index in region i, and n.sub.c is the
average refractive index of the cladding region. Deltas are
conventionally expressed as percents.
[0013] The term .alpha.-profile refers to a refractive index
profile, expressed in terms of .DELTA.(b), where b is radius, which
follows the equation
.DELTA.(b)=.DELTA.(b.sub.0)(1-[.vertline.b-b.sub.0.vertline./(b.sub.1-b.su-
b.0)].sup..alpha.)
[0014] where b.sub.0 is the point at which .DELTA.(b) is maximum,
b.sub.1 is the point at which .DELTA.(b) % is zero, and b is in the
range b.sub.i.ltoreq.b.ltoreq.b.sub.f, where delta is defined
above, b.sub.i is the initial point of the .alpha.-profile, b.sub.f
is the final point of the .alpha.-profile, and .alpha. is an
exponent which is a real number.
SUMMARY OF THE INVENTION
[0015] One aspect of the present invention relates to an optical
fiber including a core, the core including silica doped with at
least about 6 mol% germania and at least about 0.9 wt % fluorine;
and a cladding surrounding the core.
[0016] Another aspect of the present invention relates to an
optical fiber including a core, the core including silica doped
with at least about 6 mol % germania and with fluorine; and a
cladding surrounding the core, wherein the optical fiber has a
numerical aperture of less than about 0.22 at 1550 nm.
[0017] Another aspect of the present invention relates to a method
of fabricating a fiber Bragg grating, the method including the
steps of providing an optical fiber including a core, the core
including silica doped with at least about 6 mol % germanium and at
least about 0.9 wt % fluorine, and a cladding surrounding the core;
and exposing a section of the optical fiber to patterned UV
radiation, thereby writing the grating in the core of the
fiber.
[0018] The optical fibers of the present invention result in a
number of advantages over prior art optical fibers. For example,
the optical fibers of the present invention have high
photosensitivity while maintaining a desirably low numerical
aperture. The skilled artisan may essentially independently adjust
photosensitivity and numerical aperture by judiciously selecting
dopant levels. The optical fibers of the present invention may have
high glass homogeneity and uniformity, and thus low optical loss
due to scattering. Fiber Bragg gratings using the optical fibers of
the present invention may be fabricated without the use of a
hydrogen loading process. Additional features and advantages of the
invention will be set forth in the detailed description which
follows, and in part will be readily apparent to those skilled in
the art from the description or recognized by practicing the
invention as described in the written description and claims
hereof, as well as in the appended drawings.
[0019] It is to be understood that both the foregoing general
description and the following detailed description are merely
exemplary of the invention, and are intended to provide an overview
or framework to understanding the nature and character of the
invention as it is claimed.
[0020] The accompanying drawings are included to provide a further
understanding of the invention, and are incorporated in and
constitute a part of this specification. The drawings are not
necessarily to scale. The drawings illustrate one or more
embodiment(s) of the invention, and together with the description
serve to explain the principles and operation of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a cross-sectional view of an optical fiber
according to one embodiment of the present invention; and
[0022] FIG. 2 is a plot showing the results of splicing an optical
fiber of the present invention to a conventional optical fiber.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] The invention disclosed and described herein relates to an
optical fiber suitable for the manufacture of fiber Bragg gratings.
According to one aspect of the invention, an optical fiber includes
a core including silica doped with at least about 6 mol % germania
and with at least about 0.9 wt % fluorine. One embodiment of an
optical fiber of the present invention is shown in cross-sectional
view in FIG. 1. Optical fiber 20 includes a core 22, and a cladding
24. The core is substantially disposed about the centerline of the
fiber, and may have any desired refractive index profile shape,
including, for example, a step profile, a rounded step profile, a
trapezoidal profile, a rounded trapezoidal profile, or an
(.alpha.-profile. It is noted that the dopant levels recited herein
are taken at the maximum index of the profile. It will be
appreciated by the skilled artisan that the refractive index
profile may have an index depression along the centerline. The
cladding may be substantially uniform in index, and formed from a
single material, such as undoped silica or phosphorus-fluorine
co-doped silica. The cladding may alternatively include a plurality
of differently-doped layers, as would be familiar to the skilled
artisan. For example, the cladding may include a germania-fluorine
co-doped silica inner cladding and a phosphorus-fluorine co-doped
silica outer cladding. As is customary, the fiber may be coated
with one or more layers of polymer coatings 26.
[0024] According to one embodiment of the present invention, an
optical fiber is doped with at least about 6 mol % germania, and at
least about 0.9 wt % fluorine in the core of the fiber. The core of
the optical fiber may suitably be doped with at least about 7 mol %
germania. The photosensitivity of the core of the fiber will be
strongly dependent on the amount of germania. The core of the
optical fiber may suitably be doped with at least about 1.2 wt %
fluorine. The .DELTA. and the numerical aperture of the fiber will
be dependent on the relative amounts of germania, fluorine, and
other dopants in the core and the cladding. Increased amounts of
germania in the core will raise the index of the core, and
therefore increase the .DELTA. and the numerical aperture of the
fiber. Increased amounts of fluorine in the core will decrease the
index of the core, and therefore decrease the .DELTA. and the
numerical aperture. As such, the increase in .DELTA. and numerical
aperture caused by higher amounts of germania can be counteracted
by the use of higher amounts of fluorine. The skilled artisan will
adjust the amounts of germania and fluorine in the core of the
fiber to yield a desired level of photosensitivity and a desired
numerical aperture. In desirable embodiments of the invention, the
core of the optical fiber is doped with between about 0.9 wt % and
about 6 wt % fluorine; and with between about 6 mol % and about 30
mol % germania.
[0025] In one embodiment of the present invention, the core of the
optical fiber includes silica, germania, and fluorine as described
above, with no other dopants being present in the core in
substantial amounts. For example, in one embodiment of the
invention, the core of the optical fiber contains less than about
0.1 wt % of dopants other than germania or fluorine. Optical fibers
according to this embodiment of the invention may be advantageous
due to low loss and simplified manufacture.
[0026] According to another embodiment of the invention, an optical
fiber includes a core doped with fluorine and with at least about 6
mol % germania, the optical fiber having a numerical aperture of
less than about 0.22. Especially desirable optical fibers have a
numerical aperture of less than about 0.16. In designing optical
fibers according to this embodiment of the invention, the skilled
artisan can select a concentration of germania to yield a desired
photosensitivity, and select an amount of fluorine to yield a
desired numerical aperture.
[0027] According to one embodiment of the present invention, the
core of the optical fiber is substantially devoid of boron. For
example, in one embodiment of the invention, the core of the
optical fiber contains less than about 0.1 wt % boron oxide.
Inclusion of boron in a germania-doped material can cause poor
homogeneity and uniformity. Gratings fabricated in a germania-doped
fiber including boron in the core require a strong post-exposure
annealing step, making the grating fabrication process somewhat
more complex and time-consuming.
[0028] Germania concentration, fluorine concentration, and
numerical aperture for three step index optical fibers according to
one embodiment of the present invention and two conventional fibers
are shown in Table 1. The cores are formed from silica doped with
germania and fluorine as shown in Table 1. Fibers 1, 2 and 3 have
core radii of 3 .mu.m, 2.5 .mu.m, and 2 .mu.m, respectively. The
skilled artisan will recognize that the optical fibers of the
present invention may have substantially different core radii than
those of the fibers of Table 1, depending on desired optical
properties. In the fibers of Table 1, the cladding is
phosphorus-fluorine co-doped silica, and is index-matched to
undoped silica. Conventional fibers germania-doped fibers A and B
have substantially no fluorine in the core. The fibers were
fabricated using standard MCVD methods.
1TABLE 1 Numerical Germania concentration Fluorine concentration
aperture FIBER of core (mol %) of core (wt %) (1550 nm) Fiber 1 7.2
1.5 0.12 Fiber 2 7.8 1.5 0.14 Fiber 3 12.5 1.5 0.195 Conv. 5.2 0
0.14 Fiber A Conv. 9.5 0 0.195 Fiber B
[0029] Germania concentrations were measured using scanning
electron microscopy. Fluorine concentrations were computed from the
measured germania concentrations and the measured numerical
apertures of the fibers. Fibers 1, 2 and 3 of Table 1 have high
germania concentrations in their cores, yet have desirably low
numerical aperture values. In order to allow for efficient splicing
to conventional optical fibers, it is desirable for the numerical
aperture of the fibers of the present invention to be less than
about 0.22 at 1550 nm. For splicing to certain low numerical
aperture conventional optical fibers, it may be desirable for the
optical fibers of the present invention to have a numerical
aperture of 0.16 at 1550 nm. For example, fiber 1 has a numerical
aperture of 0.12 at 1550 nm, making it suitable for splicing to
SMF-28.RTM. single mode fiber, available from Coming Incorporated
of Corning, N.Y. Fiber 2 has a numerical aperture of 0.14 at 1550
nm, and is suitable for splicing to PUREMODE.TM. HI 1060 fiber,
available from Corning Incorporated of Coming, N.Y. Fiber 3 has a
numerical aperture of 0.195 at 1550 nm, and is suitable for
splicing to PUREMODE.TM. HI 980 fiber, available from Corning
Incorporated of Corning, N.Y. Fibers 2 and 3 have the same
numerical aperture values as conventional fibers A and B,
respectively, but have much higher germania concentrations.
[0030] Table 2 shows the photosensitivity of the optical fibers of
Table 1. Bragg gratings were written into the fibers without
hydrogen loading using an excimer UV source operating at a
wavelength of 244 nm. The source was pulsed at 70 Hz, with an
energy of 200 mJ/pulse. The writing energy was passed through a
variable attenuator set at 17% transmission, then though a phase
mask. The visibility of the interference fringes [(max.
intensity-min. intensity)/max. intensity] was about 80%. Calculated
refractive index changes are given for both a three minute exposure
and a saturating exposure. As used herein, a saturating exposure is
an exposure sufficient for the optical fiber to reach its maximum
UV-induced index change (i.e. a maximum Bragg grating
strength).
2TABLE 2 Refractive index change at Refractive index change FIBER
1550 nm (3 minute exposure) at 1550 nm (saturation) Fiber 1 5.8
.times. 10.sup.-4 no data Fiber 2 6.2 .times. 10.sup.-4 1.6 .times.
10.sup.-3 Fiber 3 9.2 .times. 10.sup.-4 no data Conv. Fiber A 3.7
.times. 10.sup.-4 no data Conv. Fiber B 5.1 .times. 10.sup.-4 1.5
.times. 10.sup.-3
[0031] The fibers of the present invention have higher
photosensitivities than analogous germania-doped fibers without
fluorine in the core. For example, while fiber 2 and conventional
fiber A have similar numerical apertures, the photosensitivity of
fiber 2 is over 165% that of conventional fiber A. Likewise, while
fiber 3 and conventional fiber B have similar numerical apertures,
the photosensitivity of fiber 3 is over 180% that of conventional
fiber B. Desirable fibers of the present invention exhibit an index
change at 1550 nm in the core in the absence of hydrogen loading of
at least about 5.5.times.10.sup.-4 upon exposure to a dose of
radiation having a wavelength of 244 nm and an energy of 428 J
through a phase mask yielding an interference pattern with a
visibility of about 80%. Especially desirable fibers of the present
invention exhibit an index change at 1550 nm in the core in the
absence of hydrogen loading of at least about 6.0.times.10.sub.-4
upon exposure to a dose of radiation having a wavelength of 244 nm
and an energy of 428 J through a phase mask yielding an
interference pattern with a visibility of about 80%.
[0032] A useful parameter to quantify the photosensitivity of the
optical fibers of the present invention is the ratio of the index
change at 1550 nm in the core to the numerical aperture of the
fiber, the index change being caused by an exposure in the absence
of hydrogen loading to a dose of radiation having a wavelength of
244 nm and an energy of 428 J through a phase mask yielding an
interference pattern with a visibility of about 80%. Desirable
optical fibers of the present invention have a ratio of index
change at 1550 nm in the core to numerical aperture of at least
about 3.0.times.10.sup.-3, the index change being caused by an
exposure in the absence of hydrogen loading to a dose of radiation
having a wavelength of 244 nm and an energy of 428 J through a
phase mask yielding an interference pattern with a visibility of
about 80%. Especially desirable optical fibers of the present
invention have a ratio of index change at 1550 nm in the core to
numerical aperture of at least about 4.0.times.10.sup.-3, the index
change being caused by an exposure in the absence of hydrogen
loading to a dose of radiation having a wavelength of 244 nm and an
energy of 428 J through a phase mask yielding an interference
pattern with a visibility of about 80%.
[0033] Another useful parameter to quantify the photosensitivity of
the optical fibers of the present invention is the ratio of the
index change at 1550 nm in the core in the absence of hydrogen
loading upon a saturating exposure to the numerical aperture of the
fiber. Desirable optical fibers of the present invention have a
ratio of saturated index change at 1550 nm in the absence of
hydrogen loading to numerical aperture of at least about
9.0.times.
[0034] 1. Especially desirable optical fibers of the present
invention have a ratio of saturated index change at 1550 nm in the
absence of hydrogen loading to numerical aperture of at least about
1.05.times.10.sup.-2.
[0035] The optical fibers of the present invention are designed
with numerical apertures appropriate to allow coupling to standard
optical fibers with low optical loss. Fiber 2 of Table 1 was
designed to have a numerical aperture similar to that of
PUREMODE.TM. HI 1060 optical fiber. FIG. 2 is a plot comparing the
results of splicing fiber 2 of Table 1 with PUREMODE.TM. HI 1060
with the results of splicing PUREMODE.TM. HI 1060 to itself. Eleven
splices of each combination were made. The splice parameters used
were those found in the menu of a FUJIKURA 40S fusion splicer for a
PUREMODE.TM. HI 1060-PUREMODE.TM. HI 1060 splice. The average
splice loss for fiber 2 of Table 1 with PUREMODE.TM. HI 1060 was
less than 1 dB, and was statistically similar to the splice loss
for PUREMODE.TM. HI 1060 with itself.
[0036] The optical fibers disclosed herein may be made by standard
optical fiber fabrication processes, as will be apparent to the
skilled artisan. For example, a fiber preform may be constructed
using modified chemical vapor deposition (MCVD), outside vapor
deposition (OVD), vapor axial deposition (VAD), or rod-in-tube
processes. Standard consolidation and draw processes may be used in
the fabrication of an optical fiber from the preform. Thus, the
refractive index and compositional profiles of the optical fibers
disclosed herein may be accomplished using manufacturing techniques
known to those skilled in the art including, but in no way limited
to, OVD, VAD and MCVD processes.
[0037] Another aspect of the present invention relates to a method
of fabricating a fiber Bragg grating in one of the optical fibers
described hereinabove. In one embodiment of the invention, the
method includes the steps of providing an optical fiber having a
core, the core including silica doped with at least about 6 mol %
germania and at least about 0.9 wt % fluorine, and a cladding
surrounding the core; and exposing a section of the optical fiber
to patterned UV radiation, thereby writing the grating in the core
of the fiber. The exposure is suitably performed without hydrogen
loading of the fiber. This method may be used with the optical
fibers of the present invention to make efficient fiber Bragg
gratings without the use of a hydrogen loading process.
[0038] Another aspect of the present invention includes a fiber
Bragg grating fabricated in one of the optical fibers described
hereinabove. For example, one embodiment of the invention includes
an optical fiber having a core, the core including silica doped
with at least about 6 mol % germania and at least about 0.9 wt %
fluorine, and a cladding surrounding the core, wherein a fiber
Bragg grating is present in the core of the optical fiber. Fiber
Bragg gratings of the present invention may be coupled with low
loss to other optical fibers in an optical communications
system.
[0039] It will be apparent to those skilled in the art that various
modifications and variations can be made to the present invention
without departing from the spirit and scope of the invention. Thus,
it is intended that the present invention cover the modifications
and variations of this invention provided they come within the
scope of the appended claims and their equivalents.
* * * * *